1. Field of the Invention
The present invention relates to an image forming apparatus using an electrophotographic system. For example, the present invention relates to an image forming apparatus such as a copier, a printer, and a facsimile.
2. Related Background Art
There have conventionally known image forming apparatuses using an electrophotographic system or an electrographic recording system such as a copier, a facsimile, and a printer. For example, an image forming apparatus of electrophotographic system will be described. This type of image forking apparatus obtains an image by uniformly charging a surface of, e.g., a drum-shaped electrophotographic photosensitive member (hereinafter referred to as xe2x80x9ca photosensitive drumxe2x80x9d) used as an image bearing member, then exposing the surface in accordance with image information to form an electrostatic latent image, visualizing this electrostatic latent image with developer (including toner) to form so called a toner image which is a visible image, and subsequently transferring this toner image from the photosensitive drum to a recording material and then fixing it.
Further, various image forming apparatuses having a plurality of image forming portions have been provided which form differently colored toner images in the respective image forming portions and transfer the toner images by sequentially superimposing them on the same recording material to form a color image. In this type of image forming apparatus, a recording material is fed to each image informing portion using an endless recording material bearing member. Such a color electrophotographic image forming apparatus is used for high speed recording in a color copier or the like.
FIG. 1 shows a schematic configuration of an example of an color image forming apparatus using electrophotographic system.
In the image forming apparatus of FIG. 1, first, second, third and fourth image forming portions Pa, Pb, Pc and Pd are provided together. In the respective image forming portions differently colored (cyan, magenta, yellow and black in this example) toner images are separately formed by being subjected to the respective processes of forming a latent image by charging and exposure, of forming a toner image by developing, and of transferring the image. A transfer belt 130 that is a belt-shaped recording material bearing member is provided adjacently to photosensitive drums 3a, 3b, 3c and 3d used as the image bearing members provided on the respective image forming portions while being stretched around a driving roller 13 and supporting rollers 14 and 15, and toner images of the respective colors formed on the respective photosensitive drums 3a to 3d are transferred onto a recording material S borne and fed by the transfer belt 130.
Surfaces of the photosensitive drums 3a to 3d which the image forming portions Pa to Pd have respectively are uniformly charged with drum chargers 2a, 2b, 2c and 2d which are charging means provided on the respective outer peripheries of the photosensitive drums 3a to 3d. After that, a laser beam emitted from a light source device (not shown) is scanned by turning a polygon mirror 117, the scanned light flux is deflected with a reflecting mirror in a light-guiding means 116 and is further condensed on the generatrix of the respective photosensitive drums 3a to 3d with an fxcex8 lens to expose them, and latent images according to the image information are formed on these photosensitive drums 3a to 3d. Additionally, the outer peripheries of the respective photosensitive drums 3a to 3d are also provided with light exposure lamps 111a, 111b, 111c and 111d and electric potential sensors 113a, 113b, 113c and 113d, respectively.
Developing devices 1a, 1b, 1c and 1d provided along the outer peripheries of the respective photosensitive drums 3a to 3d are replenished with a given amount of cyan, magenta, yellow and black toner, respectively, as developers by supply devices (not shown) and the respective developing devices 1a to 1d develop the electrostatic latent images formed on the respective photosensitive drums 3a to 3d as mentioned above to form a cyan toner image, a magenta toner image, an yellow toner image and a black toner image, which are visible images.
On the other hand, the recording material S is contained in a recording material cassette 10, is supplied from here onto the transfer belt 130 through a plurality of feeding rollers and registration rollers 12, and is borne and fed with the transfer belt 130 so as to be sequentially sent to transfer portions that are opposite to the photosensitive drums 3a to 3d. 
The transfer belt 130 is made of a dielectric material sheet such as polyethylene terephthalate resin sheet (PET resin), polyvinylidene fluoride resin sheet, or polyurethane resin sheet. As the transfer belt 130, an endless belt in which both ends were overlapped and joined together, or a seamless belt is used.
When this transfer belt 130 is turned with the driving roller 13 and it is confirmed the transfer belt 130 is at a given position, the recording material S is fed from the registration rollers 12 to the transfer belt so that the recording material S is fed toward the transfer portion (an opposed portion between the photosensitive drum 3a and the transfer belt 130) of the first image forming portion Pa. At the same time, an image writing signal is turned ON and an image formation is performed as described above on the photosensitive drum 3a of the first image forming portion Pa at desired timing with reference to this signal. Then, by imparting electric field or charge at the transfer portion on the lower side of the photosensitive drum 3a in FIG. 1 with transfer means 24a, a toner image of first color formed on the photosensitive drum 3a is transferred onto the recording material S.
This transfer causes the recording material S securely borne on the transfer belt 130 by electrostatic attractive force and the recording material S is fed to the second image forming portion Pb and downstream thereof.
As the transfer means 24a to 24d a non-contact charger such as corona discharge, or a contact charger using a transfer charging member such as a blade, roller or brush is used. The non-contact charger has problems that the charger generates ozone and is weak to variations in temperature and humidity circumstances of the air because of charging through air so that images are not formed stably. On the other hand, the contact charger has merits of being ozoneless, strong against the variations in temperature and humidity circumstances, capable of producing a high quality image, etc.
In the second to fourth image forming portions Pb to Pd, toner images are transferred on the recording material S while being superimposed in the same way as mentioned above. After that, the recording material S to which four colored toner images were transferred is separated from the transfer belt 130 by reducing the electrostatic attractive force by eliminating the residual charge with a separation charger 32, downstream in the feeding direction of the transfer belt 130. Usually, the driving roller 13 is grounded for performing stable separation. Further, as the separation charger 32 a non-contact charger is used because the recording material S is charged before a toner image is fixed thereto.
The recording material S to which four colored toner images have been transferred and which has been separated from the transfer belt 130 is fed to a fixing device 9 by a feeding portion 62 and is heated and pressurized at the fixing device 9 so that mixing of toner images and fixing thereof to the recording material S are performed. The fixing device 9 is composed of a fixing roller 51, a pressure roller 52, heat resistant cleaning members 54 and 55 for cleaning the rollers 51 and 52, respectively, roller heaters 56 and 57 provided in the rollers 51 and 52, respectively, an application roller 50 for applying a mold releasing oil such as dimethyl silicone oil to the fixing roller 51, a reservoir 53 for this oil, and a thermistor 58 which detects the temperature of the surface of the pressure roller 52 to control the fixing temperature.
Thus, the recording material S on which a full color image has been formed is discharged to a discharge tray 63 outside the image forming apparatus. When the transfer is completed, the photosensitive drums 3a to 3d is removed of residual untransferred toner and cleaned with cleaners 4a, 4b, 4c and 4d provided on the outer peripheries of the photosensitive drums 3a to 3d, respectively, and is subsequently prepared for image formation. Further, toner and other contaminants remaining on the transfer belt 130 are cleaned off with a cleaning web (nonwoven fabric) 19 abutting against the surface of the transfer belt 130.
Next, a transfer portion will be described in detail. In the respective image forming portions Pa to Pd, the configurations of the transfer portions are the same. Thus, the description will be made taking one transfer portion as an example, unless a special explanation on other transfer portions is needed. Incidentally, the indices a to d (as xe2x80x9caxe2x80x9d in the photosensitive drum 3a of the first image forming portion Pa) which indicate the members denoted by reference symbols including the indices belong to the respective image forming portions, are omitted.
It is conventionally known that when electric current (hereinafter referred to as xe2x80x9ctransfer electric currentxe2x80x9d) which contributes during transfer is kept constant at a proper level, an image is stabilized. Thus, it is general to perform a constant current control on transfer means so that a constant current can be obtained even when volume resistivity is changed by, for example, sorts (thickness, material and the like) of the recording material S or by moisture absorption conditions.
FIG. 2 shows a schematic cross-sectional view of the vicinity of the transfer portion. As shown in FIG. 2, if electric current in the transfer portion is defined as I (xcexcA), if the width of the recording material S in a direction (hereinafter referred to as xe2x80x9ca thrust directionxe2x80x9d) perpendicular to the movement direction of the transfer belt 130 is defined as P (cm), and if a movement speed (hereinafter referred to as xe2x80x9ca process speedxe2x80x9d) of the recording material bearing member (transfer belt 130) is defined as v (cm/sec), surface charge density xcfx81(xcexcC/cm2) on the recording material S is expressed as follows.
xcfx81=I/(Pxc3x97v)
Transfer is stably carried out by imparting charge having such a constant surface charge density p to the recording material S.
Next, voltage (hereinafter referred to as xe2x80x9ctransfer voltagexe2x80x9d) applied to a transfer portion will be described.
As described above, the transfer belt 130 is generally made of a film of so called engineering plastic such as PET (polyethylene terephthalate) or PC (polycarbonate). These plastic films are dielectric members each of which usually has volume resistivity on the order of 1016 xcexa9cm and relative permittivity of about 3 to 4.
The transfer means 24 may be a transfer blade (conductive blade) comprising plate-shaped-conductive rubber, or a transfer roller comprising a roller made of a material similar to the conductive rubber. The transfer means 24 has sufficiently low resistance as compared with the transfer belt 130 which plays the role of an electrode. In general, this conductive rubber is a conductor having volume resistivity on the order of 106 xcexa9cm.
The transfer belt 130 may be an insulator and is compared to a capacitor. If the transfer belt 130 is not being turned, the transfer current is not supplied to the transfer portion. However, when the transfer belt 130 is turning, empty capacitors reach the positions of the transfer means 24 in succession whereby the transfer means 24 charge the transfer belt 130.
That is, although the transfer belt 130 which is not being turned is a capacitor, turning of the transfer belt 130 supplies a constant current to the transfer belt 130 and the transfer belt 130 can be compared to electric resistance.
At this time to a power supply 40 which supplies transfer current in the transfer portion is applied a constant voltage (transfer voltage). FIG. 3 shows a schematic cross-sectional view of the vicinity of the transfer portion. Further, FIG. 4 shows an equivalent circuit of the transfer portion shown in FIG. 3.
Electric resistors which increase transfer voltage in the transfer portion are conventionally the transfer belt 130, the recording material S, the photosensitive drum 3 and the transfer means 24 in the order of higher resistance. In the present specification, the voltage applied to each member is defined as a partial voltage in each portion. Incidentally, the order of the electric resistors in the transfer portion are different from the order mentioned above, depending on the type of the image forming apparatus and the sorts of the recording materials S.
In FIG. 3, since a transfer blade 25 made of a conductive rubber, which configurates the transfer means 24, is not turned (moved) like the transfer belt 130 during image formation, the resistance value accounts for the partial voltage as it is. That is, if the volume resistivity of the transfer blade 25 is defined as xcfx81Vblade, if a free length of the transfer blade 25 is defined as L, and if a transfer nip (the width across which the transfer blade 25 is in contact with the transfer belt 130 in the movement direction of the transfer belt 130) is defined as d, resistance Rblade of the transfer blade 25 during transfer is expressed as follows.
Rblade=xcfx81Vbladexc2x7L/d
On the other hand, the transfer belt 130 is turned during image formation. If the volume resistivity of the transfer belt is defined as xcfx81Vbelt and if the thickness thereof is defined as t, resistance Rbelt in the case where the transfer belt 130 is in stop is expressed as follows:
Rblade=xcfx81Vbeltxc2x7t/d
Since the transfer belt 130 is actually being turned during operation of image formation, resistance of the transfer belt 130 during turning is lower than resistance during stoppage of the transfer belt 130.
In the image forming apparatus with the above structure, recording materials S having various sizes (length (length in the thrust direction) in a direction perpendicular to the feeding direction of the recording material) can be used. To carry out an appropriate transfer even when image formation is performed on a recording material S of a smaller size than the maximum size with which image formation can be carried out, it is preferable that surface charge density on the recording material S of a small size, which is passed through a transfer portion, is substantially the same as the surface charge density on the maximum size recording material S when the recording material of the maximum size is passed through the transfer portion.
FIG. 5 is a cross-sectional view schematically showing the vicinity of a transfer portion when the recording material S of a small size is passed through the transfer portion.
As shown in FIG. 5, if electric current, which flows through a passing portion of the recording material S, is defined as Ip (xcexcA), if electric current which is passed through a non-passing portion of the recording material S, is defined as Iq (xcexcA), if the width of a recording material S of a small size in the thrust direction is defined as p (cm), if the width of the non-passing portion in the thrust direction is defined as q (cm), and if a width of the transfer blade 25 constituting the transfer means 24 in the thrust direction is defined as t (cm), the following equation:
xcfx81=Ip/(pxc3x97v)=I/(txc3x97v)xe2x80x83xe2x80x83(1)
must be satisfied to equalize the surface charge density on the recording material S of a small size when the recording material S of a small size is passed through the transfer portion to the surface charge density of the maximum size recording material S when the recording material S of the maximum size is passed through the transfer portion as described above. In the above equation (1), v is a process speed (cm/s, the movement speed of the transfer belt (recording material)).
However, since the resistance of the recording material S itself is present in the passing portion, impedance in the passing portion of the recording material S is high as compared with the non-passing portion. Accordingly, the current, which is passed through a passing portion, per unit area is smaller than in a non-passing portion. That is, the surface charge density on the recording material S is small as compared with that in the non-passing portion. Therefore, the surface charge density on the recording material S becomes insufficient and transfer efficiency is decreased. As a result, sufficient image concentration cannot be obtained.
That is, in fact, the above-mentioned relation (1) cannot be obtained. The following relations are obtained instead:
Iq/(qxc3x97v) greater than I/(txc3x97v) greater than Ip/(pxc3x97v)
The passing portion of the recording material S has insufficient charge density and the non-passing portion thereof has excess charge density.
FIG. 6 shows the equivalent circuit of the transfer portion shown in FIG. 5 when a recording material S of a small size is passed through the transfer portion. The ratio of current Ip which flows to the passing portion of the recording material S to current Iq which flows to the non-passing portion thereof is inversely proportional to the resistance ratio between the passing portion of the recording material S and the non-passing portion thereof. That is, if the resistance of the passing portion of the recording material S is defined as Rp and if the resistance of the non-passing portion thereof is defined as Rq, the following equation is satisfied:
Ip:Iq=Rq:Rp
This problem is likely to occur particularly in the case where an image is transferred to a recording material S which has been dried under low humidity circumstances (for example, 23xc2x0 C., 5% RH) to exhibit high resistance. In general, the volume resistivity of the recording material S is varied on the order of from 1xc3x97107 to 1xc3x971014 xcexa9cm, depending upon the sorts of the recording material S and temperature and humidity conditions. However, the transfer belt 130 is made of a dielectric resin sheet and the volume resistivity thereof is varied by about 1 to 3 digits, utmost due to the temperature and humidity conditions. Further, in the transfer belt 130 with low volume resistivity, whose resistance is adjusted to a low level by additives, this phenomenon is even more likely to occur.
As described above, when the recording material S of a small size is passed through the passing portion, a large amount of current is supplied to the non-passing portion that has small resistance and current which flows into the passing portion is insufficient thereby to cause a transfer failure.
Further, since excess current which then flows into the non-passing portion of the recording material S flows to the photosensitive drum 3, a so-called transfer memory remains on the photosensitive drum 3 whereby fogging (unnecessary toner adhesion) might take place on an image.
Furthermore, when excess current flows under high temperature and high humidity circumstances, charge is penetrated into the recording material S to charge negative charging toner on the surface of the photosensitive drum 3 to an inverse polarity, thereby causing a transfer failure. As a result, a phenomenon of lowering the image density occurs.
An object of the present invention is to provide an image forming apparatus which can stably, excellently form images on recording materials of various sizes.
Another object of the present invention will become apparent by reading the following detailed descriptions.